We're in the process of updating the documentation for Zotero 5.0. Some documentation may be outdated in the meantime. Thanks for your understanding.
As you build your Zotero library, you might introduce a few duplicated items. E.g. you could have saved the same item twice from a webpage, or imported items already in your library. Fortunately, Zotero can help you identify possible duplicates and allow you to merge them.
Clicking on the “Duplicate Items” collection in your library or right-clicking the library in the left column and selecting “Show Duplicates” will show the items Zotero thinks are duplicates in the center column.
Zotero currently uses the title, DOI, and ISBN fields to determine duplicates. The algorithm will be improved in the future to incorporate other fields.
In the “Duplicate Items” collection, select an item in the center column. Zotero will automatically co-select the other items that it thinks are duplicates. Click the “Merge n Items” button in the right column to merge the items.
You can also select a group of two or more items of the same item type anywhere in your Zotero library, right-click, and select “Merge Items…” from the context menu.
Merges are automatically recognized by the word processor plugins and don't affect your automatically generated citations and bibliographies.